Enter Amity Square on the East side by the Gym. Then go to the ruins but do not enter. Go to the left side of the ruins and enter the portal where you will end up by a girl with a Happiny. Go to the ruins and go to the left again and you will find a man who gives you items that he finds. Go to the ruins and go to the right side to find the Spooky Plate.
Hope I Helped